The present invention relates to a register error amount detection method and apparatus which obtain register error amounts between the colors of patterns printed on a printing product based on the positions of register marks.
For example, in an offset printing press, a printing plate on which patterns are printed is wound around a plate cylinder, and ink retained in an ink fountain is transferred onto the printing plate via an ink roller group. The ink transferred onto this printing plate is transferred to a blanket cylinder which rotates in tight contact with the plate cylinder, and a printing sheet is fed through between this blanket cylinder and an impression cylinder, thereby printing on the sheet. When performing multicolor printing, a plurality of printing units each including an ink fountain, an ink roller group, a plate cylinder, a blanket cylinder, and an impression cylinder are disposed for the respective colors, and a printing sheet is sequentially fed to the printing units for the respective colors to perform printing.
In this multicolor printing press, if the position of the printing plate wound around the plate cylinder of the printing unit for each color is shifted from a proper position, positional shifts (register errors) occur between patterns of the respective colors on a finished printing product in the vertical direction (the printing direction (the traveling direction of the printing product) in the printing press) and the horizontal direction (the direction perpendicular to the vertical direction).
Under the circumstance, Japanese patent Laid-Open No. 2011-110885 (literature 1) discloses a printing press which performs inline registration as follows. When printing patterns of a plurality colors, this printing press simultaneously prints a register mark of each color in a margin of a printing product, and captures an image of a region including the register mark of each color by using a high-resolution camera. The printing press obtains the position of the register mark of each color in the captured image data, and obtains the register error amounts between the colors of the patterns printed on the printing product based on the obtained positions of the register marks of the respective colors. The printing press then adjusts the rotational phase of the plate cylinder, the horizontal position of the plate cylinder, and the like in the printing unit for each color based on the obtained register error amounts between the colors.
This method, however, requires a dedicated camera, and hence requires expensive equipment. In contrast to this, studies have been conducted to use the camera of a printing quality inspection apparatus. According to this technique, the position of a register mark of each color is detected from image data captured by this camera, and register error amounts between colors are obtained. The printing quality inspection apparatus is designed to capture with a camera an image of a region including an overall pattern region on a printing product on which patterns of a plurality of colors are printed and inspect the quality of each pattern printed on the printing product by using the captured image data of each pattern. Japanese patent Laid-Open No. 2012-68080 (literature 2) discloses such a printing quality inspection apparatus. Japanese patent Laid-Open No. 2006-525144 (literature 3) discloses a technique of detecting register marks by using the camera of a printing quality inspection apparatus.
As described, however, the camera used in a printing quality inspection apparatus needs to capture an image of an overall pattern region on a printing product, and hence needs to capture an image in a wide range. Inevitably, therefore, this camera performs image capturing at a wide angle. As a result, a large chromatic aberration, image distortion, and the like occur in a range near an end portion of a lens. For this reason, in a captured image, each side of a register mark of each color blurs to result in failure to accurately detect register error amounts between the respective colors. This problem will be described by using a registration measurement mark MR shown in FIG. 20.
The registration measurement mark MR is constituted by a black register mark R1, a cyan register mark R2, a magenta register mark R3, and a yellow register mark R4. The register marks R1 to R4 are printed at predetermined intervals in the vertical or horizontal direction of a printing product. In this case, “black” is a color as a reference (to be also referred to as a reference color hereinafter), and the register marks R1 to R4 are printed at predetermined intervals in the vertical direction (Y direction).
The registration measurement mark MR is used as follows. The positions of the register marks R1 to R4 of the respective colors are detected. Relative distances L1, L2, and L3 between the register mark R1 of the reference color and the register marks R2, R3, and R4 of the remaining colors in the vertical direction (Y direction) are obtained. Register error amounts Δy1, Δy2, and Δy3 of the remaining colors with respect to the reference color in the vertical direction (Y direction) are obtained from the differences between the obtained relative distances L1, L2, and L3 in the vertical direction (Y direction) and predetermined reference distances L1r, L2r, and L3r. Likewise, although not shown in FIG. 20, register error amounts Δx1, Δx2, and Δx3 of the remaining colors with respect to the reference color in the horizontal direction (X direction) are obtained. The register error amounts of the remaining colors with respect to the reference color obtained in this manner correspond to the register error amounts between the respective colors.
When capturing an image of the registration measurement mark MR by using the camera of a printing quality inspection apparatus, the camera of the printing quality inspection apparatus performs image capturing at a wide angle. This causes a large chromatic aberration and image distortion in a range near an end portion of the lens. For this reason, each side of each of the register marks R1 to R4 of the respective colors constituting the registration measurement mark MR blurs. This makes the detected positions of the register marks R1 to R4 of the respective color inaccurate, resulting in a reduction in the accuracy of the register error amounts between the respective colors which are obtained from the detected positions of the register marks R1 to R4 of the respective colors.